Optical system of optical information recording/reproducing apparatus

ABSTRACT

An optical system of an optical information recording/reproducing apparatus including a light source for emitting a generally parallel luminous flux; an objective optical system for converging the luminous flux emitted from the light source onto a medium; a beam splitter for splitting the luminous flux reflected by the medium from a light path directed to the light source and guiding the same to a light receiving system; a chromatic aberration correcting element having almost no power disposed between the objective lens and the beam splitter and adapted to correct a chromatic aberration of the objective lens; and means for independently actuating the objective lens at least in an optical axis direction thereof.

This application is a continuation of application Ser. No. 07/477,464,filed Feb. 9, 1990, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical system of an optical informationrecording/reproducing apparatus in which a semiconductor laser is usedas a light source, and further relates an objective optical system and achromatic aberration correcting element suitable to the optical system.

2. Description of the Prior Art

An optical system of an optical information recording/reproducingapparatus such as optical disk apparatus, etc. comprises, as generallyshown in FIG. 52, a light source portion 10 for emitting a generallyparallel luminous flux, an objective optical system 20 for converging(focusing) the luminous flux emitted from the light source 10 onto anoptical disk OD, a beam splitter 30 for splitting the luminous fluxdisposed between the light source 10 and the objective optical system 20and adapted to split the luminous flux reflected by the disk, and asignal detecting optical system 40 for receiving such split luminousflux.

The light source portion 10 has a semiconductor laser 11, a collimatorlens 12, and a beam shaping element 13.

The objective optical system 20 includes an objective lens 21, and amirror 22, and disposed within a head 50 which is slided in the radialdirection of the optical disk. Also, the objective lens 21 is mounted onan actuator (not shown) disposed within the head 50 and designed suchthat the objective lens 21 can be finely moved at least in the opticalaxis direction thereof so that a out of focus caused by warping of thedisk, etc. can be corrected.

The signal detecting optical system 40 has a beam splitter 41, atracking signal detecting system 42, and a focusing signal detectingsystem 43, and is adapted to reads information recorded in the disk anderror signals of the trucks by reflected light from an optical disk OD.

By the way, an emitting light wavelength of the semiconductor laser usedas a light source is shifted by changing of output power power and/ortemperature. Because of the foregoing reason, when the chromaticaberration of the optical system is not corrected yet, the position of alight converging point is varied by shifting the wavelength.

When the light converging position is not coincident with the recordingsurface of the disk, there is a high possibility that incorrect writingand reading are effected.

However, out of focus due to comparatively gentle change of a wavelengthcaused by change of temperature or the like is automatically correctedby the afore-mentioned focusing servo when the collimator lens iscorrected in chromatic aberration and change of temperature.

However, at the time when a data is written, an oscillation wavelengthof a semiconductor laser is instantaneously shifted by several nmbetween a region where the temperature is increased and a region wherethe temperature is not increased. And the out of focus caused by suchradical shift cannot be corrected by the above-mentioned focusing servo.

Therefore, especially when writing is effected, correction of thechromatic aberration of the objective optical system is important.

An optical system in which the objective lens itself is corrected inchromatic aberration is disclosed in, for example, Japanese Patent EarlyLaid-open Publication No. Sho 63-10118, Japanese Patent Early Laid-openPublication No. Sho 60-232519 and Japanese Patent Early Laid-openPublication No. Sho 58-72114.

However, the lens of the Japanese Patent Early Laid-open Publication No.Sho 63-10118 is of three pieces structure including an aspherical lens,while the lenses of the Japanese Patent Early Laid-open Publication No.Sho 60-232519 and the Japanese Patent Early Laid-open Publication No.Sho 58-72114 are of four pieces structure of glass lenses. Accordingly,there are such problems as that these lenses are heavy in weightcompared with lenses which are not corrected in chromatic aberration,and a load incurred to a movable actuator is large.

As an objective lens for an optical disk apparatus is moved at a highfrequency for the purposes of focusing and trucking, it is stronglydemanded that the objective lens is made compact in size and light inweight in order to reduce the burden to the actuator.

Also, Japanese Patent Early Laid-open Publication No. Sho 62-269922discloses an optical system for correcting the chromatic aberration ofan objective lens by excessively correcting the chromatic aberration ofa collimator lens. With this construction, it is necessary toexcessively correct even a focusing error detecting optical systembecause otherwise out of focus is generated caused by a focusing servo.

However, the correcting amount of the chromatic aberration of thefocusing error detecting optical system is proportional to a secondraised power of the ratio M between a focal length of a condenser lensof this optical system and a focal length of the objective lens.Therefore, in an ordinary optical disk apparatus taking a value of aboutM=10 in view of the size of a light receiving element, it is difficultto design as such that the condenser lens has a sufficient correctingamount of chromatic aberration.

SUMMARY OF THE INVENTION

This invention has been accomplished in order to solve theabove-mentioned problems.

An optical system of an optical information recording/reproducingapparatus according to the present invention comprises a light sourceportion for emitting a generally parallel luminous flux, an objectiveoptical system for converging the luminous flux emitted from the lightsource onto a medium, and a beam splitter for splitting the luminousflux reflected by the medium from a light path directed to the lightsource portion and guiding the same to a light receiving system, saidobjective optical system including an objective lens having a positivepower and independently driven at least for focusing and a chromaticaberration correcting element having almost no power and disposedbetween the objective lens and the beam splitter in order to correct thechromatic aberration of the objective lens.

Regarding aberration other than chromatic aberration, it is desirablethat the objective lens and the chromatic aberration correcting elementare corrected independently. The reason is that if it is constructed insuch a manner as that the aberration is offset by the objective lens andthe chromatic aberration correcting element, an aberration is generatedwhen a relative position is changed by trucking and/or focusing.

The chromatic aberration correcting element is constructed of acombination of a positive lens with a negative lens having a differentAbbe number in order to correct chromatic aberration. In order toincrease the corrected amount of the chromatic aberration, it isdesirable that these lenses are cemented with each other. The reason isthat if a spatial distance exists between the positive lens and thenegative lens, a total reflection is occurred at the peripheral portionthereby to generate an eclipse, and an aberration fluctuation isoccurred when a distance error is taken place.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a light path diagram showing a first embodiment of an opticalinformation recording/reproducing apparatus according to the presentinvention.

FIG. 2 is a diagram showing the operation of a tracking servo and aradial servo.

FIG. 3 is a light path diagram showing a second embodiment of an opticalsystem of an optical information recording/reproducing apparatusaccording to the present invention.

FIG. 4 is a light path diagram showing a third embodiment of an opticalsystem of an optical information recording/reproducing apparatusaccording to the present invention.

FIG. 5 is a lens diagram showing a concrete example of an objectivelens.

FIG. 6 are various aberration diagrams of the objective lens shown inFIG. 5.

FIG. 7 are wave aberration diagrams of the objective lens shown in FIG.5.

FIG. 8 is a graph showing the movement of a light converged positioncaused by wavelength fluctuation of the lens shown in FIG. 5.

FIG. 9 is a lens diagram showing EXAMPLE 1 of an objective opticalsystem.

FIG. 10 are various aberration diagrams of the objective optical systemshown in FIG. 9.

FIG. 11 are wave aberration diagrams of the objective optical systemshown in FIG. 9.

FIG. 12 is a lens diagram showing EXAMPLE 2 of an Objective opticalsystem.

FIG. 13 are various aberration diagrams of the objective optical systemshown in FIG. 12.

FIG. 14 are wave aberration diagrams of the objective optical systemshown in FIG. 12.

FIG. 15 is a lens diagram showing EXAMPLE 3 of an objective opticalsystem.

FIG. 16 are various aberration diagrams of the objective optical systemshown in FIG. 15.

FIG. 17 are wave aberration diagrams of the objective optical systemshown in FIG. 15.

FIG. 18 is a lens diagram showing EXAMPLE 4 of an objective opticalsystem.

FIG. 19 are various aberration diagrams of the objective optical systemshown in FIG. 18.

FIG. 20 are wave aberration diagrams of the objective optical systemshown in FIG. 18.

FIG. 21 is a lens diagram showing EXAMPLE 5 of an objective opticalsystem.

FIG. 22 are various aberration diagrams of the objective optical systemshown in FIG. 21.

FIG. 23 are wave aberration diagrams of the objective optical systemshown in FIG. 21.

FIG. 24 is a lens diagram showing EXAMPLE 6 of an objective opticalsystem.

FIG. 25 are various aberration diagrams of the objective optical systemshown in FIG. 24.

FIG. 26 are wave aberration diagrams of the objective optical systemshown in FIG. 24.

FIG. 27 are wave aberration diagrams by a single unit of the objectivelens shown in FIG. 24.

FIG. 28 are wave aberration diagrams by a single unit of the objectivelens shown in FIG. 24.

FIG. 29 is a lens diagram showing EXAMPLE 7 of an objective opticalsystem.

FIG. 30 are various aberration diagrams of the objective optical systemsshown in FIG. 29.

FIG. 31 are wave aberration diagrams of the objective optical systemshown in FIG. 29.

FIG. 32 are various aberration diagrams of a single unit of theobjective lens shown in FIG. 29.

FIG. 33 are wave aberration diagrams by a single unit of the objectivelens shown in FIG. 29.

FIG. 34 is a lens diagram showing example 8 of the objective opticalsystem.

FIG. 35 are various aberration diagrams of the objective optical systemshown in FIG. 34.

FIG. 36 are wave aberration diagrams of the objective optical systemshown in FIG. 34.

FIG. 37 is a lens diagram showing EXAMPLE 9 of the objective opticalsystem.

FIG. 38 are various aberration diagrams of the objective optical systemshown in FIG. 37.

FIG. 39 are wave aberration diagrams by a single unit of the objectivelens shown in FIG. 37.

FIG. 40 are various aberration diagrams by a single unit of theobjective lens shown in FIG. 37.

FIG. 41 are wave aberration diagrams by a single unit of the objectivelens shown in FIG. 37.

FIG. 42 is a lens diagram showing a first example of an objectiveoptical system in which a hologram lens is used as an objective lens.

FIG. 43 is a lens diagram showing a second example of an objectiveoptical system in which a hologram lens is used as an objective lens.

FIG. 44 are various aberration diagrams in a case where affection by anadhesive of the objective optical system of EXAMPLE 1 is taken intoconsideration.

FIG. 45 is a lens diagram showing EXAMPLE 10 of the objective opticalsystem.

FIG. 46 are various aberration diagrams in a case where affection by anadhesive by the objective optical system shown in FIG. 45.

FIG. 47 are various aberration diagrams wherein an adhesive is not takeninto consideration.

FIG. 48 is a lens diagram showing EXAMPLE 11 of an objective opticalsystem.

FIG. 49 are aberration diagrams showing various aberration diagramsafter taking into consideration of the objective optical system shown inFIG. 48.

FIG. 50 is a lens diagram showing EXAMPLE 12 of the objective opticalsystem.

FIG. 51 are various aberration diagrams taking into consideration theaffection by an adhesive of the objective optical system shown in FIG.50.

FIG. 52 is a light path diagram showing an optical system of theconventional optical information recording/reproducing apparatus.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The embodiment of the present invention will now be describedhereinafter with reference to the drawings. The order of the descriptionis as follow.

(1) Example of the construction of a whole optical system of an opticalinformation recording/reproducing apparatus

(2) Concrete example of an objective lens

(3) Concrete examples of an objective optical system Ex. 1 to Ex. 12

(1) The construction of a whole optical system of an optical informationrecording and reproducing apparatus Example 1

FIG. 1 shows EXAMPLE 1 of an optical system of an optical informationrecording/reproducing apparatus.

This optical system includes a light source 10, an objective opticalsystem 20, a beam splitter 30, and a signal detecting optical system 40.The light source 10 comprises a semiconductor laser 11 for generating adivergent luminous flux, a collimator lens 12 for collimating thedivergent luminous flux, a beam shaping optical system 13 for shapingthe sectional configuration of the luminous flux, thereby to generate aparallel beam of a circular shape in section.

The objective optical system 20 includes an objective lens 21 forconverging beam onto the recording surface of the optical disk OD, amirror 22, a chromatic aberration correcting element 23 for correctingthe movement of the light converged position caused by wavelength shiftof the semiconductor laser 11. The objective lens 21 and the mirror 22are disposed within a head 50 which is slided in the radial direction ofthe optical disk. The chromatic aberration correcting element comprisesa positive lens and a negative lens cemented together and is fixed tothe outside of the head 50. Also, the objective lens 21 is mounted on anactuator (not shown) which is disposed within the head 50 and can befinely moved in the optical axis direction thereof and the radialdirection of the disk.

The diameter of the luminous flux, which transmits through the chromaticaberration correcting element 23, is preferably set to be larger thanthe diameter of the objective lens 21. Because, a sufficient luminousflux can be made incident to the objective lens 21 even when theobjective lens 21 independently moved by tracking.

Both the head 50 and the objective lens 21 are actuated in the radialdirection of the disk. The actuation of the head 50 is a coarseactuation (radial servo) crossing the track, while the actuation of theobjective lens 21 is a fine actuation (tracking servo) having a highfrequency.

The signal detecting optical system 40 has a beam splitter 41, atracking signal detecting system 42, and a focusing signal detectingsystem 43 and reads the information recorded in the disk as well asrespective error signals of the focus and track by the reflected lightfrom the optical disk OD.

The actuator provided with the objective lens 21 applies a focusingservo in order to correct a out of focus caused by warping of the diskin accordance with the focus error signal and applies a tracking servoso that the spot which is converged by the objective lens 21 would notbe brought out of the track in accordance with the track error signal.

The tracking servo may employ, besides the method for actuating theobjective lens 21 as mentioned above, a method for actuating the mirror22 or the entire head 50 at a high frequency.

FIG. 2 is a diagram for explaining differences between the radial servoand the tracking servo taking a reproducing optical disk as one example.A track T shown by one-dotted chain line is formed in a spiral orconcentic shape on the optical disk OD, and a pit P is formed on thetrack T. The radial servo is a control means for moving a spot Sconverged by the objective lens across the track T as shown in the arrowin the drawing. On the other hand, the tracking servo is a control meansfor moving a spot S tracing on the track T within a fine range so thatthe spot S would not be brought out of the track.

The chromatic aberration correcting element 23, as shown in FIG. 3, maybe disposed within the head 50 as shown in FIG. 3.

When the chromatic aberration correcting element 23 is disposed outsidethe head, the head can be miniaturized. On the other hand, when thechromatic aberration correcting element is disposed within the head, theeffective aperture of the chromatic aberration correcting element can bemade smaller than the case where the chromatic aberration correctingelement is disposed outside the head because no positional displacementof an incident pupil caused by the radial servo is occurred.

Example 2

FIG. 4 shows the second embodiment of an optical system of an opticalinformation recording/reproducing apparatus.

This optical system is designed such that the semiconductor laser 11,the collimator lens 12, the beam splitter 30, the objective lens 21, thechromatic aberration correcting element 23, and the signal detectingoptical system 40 are all disposed within the head 50 which is slided inthe radial direction of the disk.

The objective lens 21 is disposed on an actuator 51 and is capable offinely moving in the optical axis direction thereof and the radialdirection of the disk.

The chromatic aberration element 23 comprises two negative lenses andone positive lens cemented together.

(2) Objective Lens

FIG. 5 shows the above-mentioned objective lens, and FIGS. 6 and 7 showthe aberration of the single unit of the objective lens. The referencecharacter OD in FIG. 5 denotes a cover glass for covering the recordingsurface of the disk.

The objective lens of the optical disk apparatus is indispensable tohave a convex surface in order to exhibit a strong converging force forconverging the luminous flux to the recording surface of the disk. Andin order to maintain the converging efficiency high, it is necessary tofully correct the spherical aberration and the coma aberration.

In order to restrain the coma aberration, it is necessary to satisfy thesine condition. To this end, it is necessary to provide a strong convexconverging surface on the light source side. This strong convergingsurface is preferably disposed near the disk in order to obtain aworking distance.

This objective lens is formed into an aspherical lens having a largerradius of curvature as it goes toward the peripheral portion thereof inorder to correct the spherical aberration and the coma aberration by asingle piece of lens and also in order to obtain a sufficient edgethickness necessary for processing while restraining the centralthickness thereof.

Concrete numerical constructions are as shown in Table 1 and Table 2. Inthe Tables, the reference character r the radius of curvature of asurface, d a lens thickness or a spatial distance, n₇₈₀ a refractiveindex in a wavelength of 780 nm of a lens, n_(d) a refractive index in ad-line (wavelength of 588 nm) of a lens, and ν_(d) an Abbe number. Thesurface No. 1 and 2 denotes the objective lens and the surface No. 3 and4 denotes the cover glass of the optical disk. Regarding the glassmaterial, the objective lens is a polymethylmethacrylate and the coverglass OD of the optical desk is BK7.

The aspherical first and second surfaces are expressed as follows;##EQU1## wherein X is a distance from a tangential plane of the vertexof an aspherical surface on the aspherical surface where the height Yfrom the optical axis, C is the radius of curvature (1/r) of the vertexof the aspherical surface, K is the coefficient of a circular cone, andthe A₄ ˜A₁₀ are aspherical surfaces coefficients. These coefficients areas shown in Table 2.

                  TABLE 1                                                         ______________________________________                                        surface                                                                       NO.   r          d       n.sub.780                                                                              n.sub.d                                                                             ν.sub.d                            ______________________________________                                        1     2.005      2.080   1.48479  1.49186                                                                             57.4                                  2     -5.231     1.390                                                        3     ∞    1.200   1.51072  1.51633                                                                             64.1                                  4     ∞                                                                 ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        first surface   second surface                                                ______________________________________                                        K = -0.5223E + 00                                                                             K = -0.3168E + 01                                             A.sub.4 = -0.1400E - 03                                                                       A.sub.4 = 0.1740E - 01                                        A.sub.6 = -0.4966E - 04                                                                       A.sub.6 = -0.4011E - 02                                       A.sub.8 = 0.1654E - 04                                                                        A.sub.8 = 0.5593E - 03                                        A.sub.10 = -0.1292E - 04                                                                      A.sub.10 = -0.3494E - 04                                      A.sub.12 = 0.0000E + 00                                                                       A.sub.12 = 0.0000E + 00                                       ______________________________________                                    

(3) Concrete example of objective optical system

Deterioration of wave aberration based on the out of focus of the singlebody of the objective lens is as shown in FIG. 8. It will be understoodfrom FIG. 8 that when the wavelength is shifted by 5 nm, a waveaberration of about 0.04λ is generated. In order to maintain theefficiency as an objective lens, the limit of the wave aberration isabout 0.05λ. Actually, however, as there exists out of focus based onfactors other than the chromatic aberration, there is a possibility thatthe above limit is exceeded by shift of the wavelength of about 5 nm.

It is the chromatic aberration correcting element which corrects the outof focus of the objective lens caused by such wavelength fluctuation asmentioned above.

It is necessary for the chromatic aberration correcting element to bechanged in power with respect to the change in wavelength in thedirection for offsetting the change of power caused by the wavelengthfluctuation of the objective lens.

In general, a lens using refraction which is not corrected in chromaticaberration takes a negative value in power change ratio Δφ/Δλ (Δφ:change of power, Δλ: shift of wavelength). Accordingly, the chromaticaberration correcting element is necessary that Δφ/Δλ takes a positivevalue.

The chromatic aberration correcting element is required to have almostno power so that the aberration would not be changed due to change inrelative position between the chromatic aberration correcting elementand the objective lens in the optical axis direction.

Also, in order to eliminate the fluctuation in aberration due todisplacement toward outside the optical axis of the chromatic aberrationcorrecting element and the objective lens, it is required to have almostno spherical aberration. The displacement is occurred due to, forexample, positional error when mounting, horizontal displacement whenfocusing, tracking, etc.

When these conditions are satisfied, there can be finally constructed anobjective optical system having no chromatic aberration even if thechromatic aberration correcting element is disposed to any positionbetween the objective lens and the beam splitter.

In order to satisfy the above requirements, the chromatic aberrationcorrecting elements shown in the following embodiments satisfy theconditions set forth hereunder.

    ______________________________________                                        |n.sub.p - n.sub.n | × 10.sup.5                                               . . . (1)                                             (n.sub.p780  - 1) (1 - ν.sub.n780 /ν.sub.p780) > 0.2                                            . . . (2)                                             (Δn.sub.p /Δ λ - Δ n.sub.n /Δ λ)        × λ.sup.2 > 9.0 nm                                                                       . . . (3)                                             |f.sub.p /f.sub.c | < 0.01                                                          . . . (4)                                             |r.sub.a /r.sub.m | > 5                                                             . . . (5)                                             |r.sub.1 /f| > 7                                                                    . . . (6)                                             |r.sub.3 /f| > 7                                                                    . . . (7)                                             ______________________________________                                    

However, the symbolic characters used in the relations have thefollowing meanings.

n_(p) : refractive index of a positive lens in center use wavelength λ

n_(n) : refractive index of a negative lens in center use wavelength λ

n_(n780), n_(n830) : refractive indexes in wavelengths 780 nm, 830 nm

n_(p780), n_(p830) : refractive indexed in wavelengths 780 nm, 830 nm

ν_(n780) : dispersion of a negative lens in the vicinity of wavelength780 nm wherein; ν_(n780) =n_(n780) /(n_(n780) -n_(n830))

ν_(p780) : dispersion of a positive lens in the vicinity of wavelength780 nm wherein; ν_(p780) =n_(p780) /(n_(p780) -n_(p830))

Δn_(p) /Δλ: gradient with respect to wavelength of a refractive index ofa positive lens

Δn_(n) /Δλ: gradient with respect to wavelength of a refractive index ofa negative lens

f_(p) : focal length of a positive lens

f_(c) : focal length in its entirety

r_(m) : radius of curvature of cemented surfaces

r_(a) : radius of curvature of noncemented surfaces of a positive lens

r₁ : radius of curvature of incident surface

r₃ : radius of curvature of outgoing surface

f: focal length of whole objective optical system

Regarding the chromatic aberration correcting element, the smaller theradius of curvature of the cemented surfaces is and the larger thedifference in positive and negative refractive indexes is, the moresignificant the generation of the aberration becomes. As the elementitself does not have power, when the aberration is generated in thecemented surface, it is difficult to correct the aberration within theelement. Accordingly, it becomes necessary to restrict the generation ofaberration at the cemented surfaces as much as possible.

In order to generates the aberration, there are means for making theradius of curvature large and means for making the difference inrefractive indexes small. However, when the cemented surfaces arebrought very close to a surface, the original function as to correct thechromatic aberration cannot be exhibited. Accordingly, there is a limitin reduction of the aberration caused by the former means. On thecontrary, when the refractive indexes are made almost equal, thegeneration of the spherical aberration and the coma aberration can berestrained even when the radius of curvature becomes considerably. Bydifferentiating the dispersion, it becomes possible to apply a change inchromatic aberration.

The relation (1) shows the condition for restraining the difference ofrefractive index of positive and negative lenses of the chromaticaberration correcting element and reducing the generation of otheraberrations than the chromatic aberration as much as possible.

However, even in the case that the condition of the relation (1) issatisfied, it is desirable that the radius of curvature of the cementedsurfaces is as large as possible. The reason is that when the radius ofcurvature of the cemented surfaces is small, the thickness of the wholechromatic aberration correcting element becomes large in order to obtainthe edge thickness of the positive lens, while when a lens having alarge numerical aperture (NA) is used, a spherical aberration of ahigher order is generated. Therefore, the chromatic aberrationcorrecting element is necessary to be formed of a combination ofmaterials capable of increasing the radius of curvature of the cementedsurfaces as much as possible but within a limit able to exhibit achromatic aberration correcting effect.

The relation (2) shows the condition for regulating the dispersion ofquality of a chromatic aberration correcting element in order to satisfythe chromatic aberration correcting effect when a semiconductor laserwhich emits a near infrared radiation (780 nm to 830 nm) is used. Incase this condition is not satisfied, even if an objective lens havingthe smallest dispersion CaFK95 (Merchandise Name: Sumida Kogaku) amongraw materials for the use of an aspherical lens obtainable at present,the chromatic aberration correcting element becomes too thick in orderto sufficiently correct the chromatic aberration, thus resulting in aproblem in respect of weight or space.

In general, if the border surface of a medium having a differentrefractive index is a curved surface, this border surface has power.Also, in case the chromatic aberration is not corrected, the power ofthe border surface is changed in accordance with the variation of thewavelength. The Variation Δφ/Δλ of the power of the cemented surfacescaused by the fluctuation of the wavelength is given by the followingrelation:

    Δφ/Δλ=(1/r.sub.m){(Δn.sub.p /Δλ)-(Δn.sub.n /Δλ)}

As the chromatic aberration amount CA of the objective lens which is notcorrected in chromatic aberration is proportional to about λ⁻², alsoΔφ/Δλ of the chromatic aberration correcting element is desirous to beproportional to λ⁻².

Accordingly, (Δn_(p) /Δλ-Δn_(n) /Δλ)×λ² becomes a value showing thechromatic aberration correcting effect of the chromatic aberrationcorrecting element.

The relation (3) stipulates a combination of materials of the chromaticaberration correcting element for satisfying the above-mentionedchromatic aberration correcting effect. In case the condition of therelation (3) is not satisfied, even if the chromatic aberration iscorrected by somehow strengthening the curvature of the cementedsurfaces, compatibility of the sufficient chromatic aberrationcorrection of the objective lens and the prevention of other aberrationdeterioration becomes impossible because the convergence on the cementedsurfaces occurred when the wavelength change is generated or the changeof dispersing degree becomes too large.

Take an optical glass of Kabushiki Kaisha Ohara for example. There arethe following combinations of glasses which satisfy the conditions ofthe relations (1) and (3) at the wavelength of, for example, 780 nm.

    ______________________________________                                        lens       lens          (1)    (3)                                           ______________________________________                                        LaSK01     SFS5          85.0   13.7                                          LaK09      SF13          16.6   12.5                                          LaK13      SF15          24.8   10.2                                          LaK08      SF15          -0.6   10.1                                          ______________________________________                                    

The relation (4) determines a ratio between the focal length f_(c) ofthe chromatic aberration correcting element and the focal length f_(p)of a positive lens of the chromatic aberration correcting element. Whenthis condition is not satisfied, if a chromatic aberration correctingamount is sufficiently provided, an apparent light source position whenlooked from the objective lens is greatly changed depending on whetherthe chromatic aberration correcting element is provided. Therefore, itis necessary to separately design the objective lens depending onwhether the chromatic aberration correcting element is provided. In thecase the chromatic aberration correcting element and the objective lensare not arranged in proximate with each other, the working distancebecomes difficult to obtain when the ratio exceeds 0.01, while a largesize of the objective lens is invited when the ratio is smaller than-0.01.

It is desirous that the incident surface and the outgoing surface of thechromatic aberration correcting element have almost no power. However,the incident and outgoing surfaces are not necessarily formed into aperfect plane respectively. In the case that these surfaces have acurvature, the surface reflected light of the chromatic aberrationcorrecting element does not become a return light to the semiconductorlaser. This is effective in preventing the signal from be adverselyaffected.

The relation (5) stipulates the radius of curvature of the cementedsurface and the non-cemented surface of the positive lens in view of theabove. The relations (6) and (7) stipulate the ratio between the radiusof curvature of the both surfaces of the chromatic aberration correctingelement and the focal length. When these conditions are not satisfied,the power of the incident and outgoing surfaces becomes large. As aresult, an aberration is easy to occur due to failing when arranged.Even if the total power is 0, it has an angle magnification. As aresult, increase of the diameter of the lens and reduce of the workingdistance are invited.

Next, examples of concrete numerical constructions of the objectiveoptical system including a chromatic aberration correcting element willbe described. In the drawing, an objective optical system formed of acombination of the chromatic aberration correcting element with theobjective lens is shown. The aberration is for the whole objectiveoptical system.

EXAMPLE 1

FIG. 9 shows EXAMPLE 1 of an objective optical system. The numericalconstruction of the chromatic aberration correcting element is shown inTable 3. In the table, the reference character NA denotes the numericalaperture, f denotes a focal length of the objective optical system in awavelength of 780 nm, ω denotes a half field angle. As the numericalvalue constructions for the objective lens and the cover glass of theoptical disk is the same to that of EXAMPLE 1, description will beomitted in the following Table.

                  TABLE 3                                                         ______________________________________                                        NA 0.55  f = 3.30  ω = 1.7°                                      surface                                 glass material                        NO.   r        d      n.sub.780                                                                           n.sub.d                                                                             ν.sub.780                                                                        name                                  ______________________________________                                        1     ∞  1.600  1.72437                                                                             1.73400                                                                             1071  LaK09                                 2     -2.200   0.800  1.72421                                                                             1.74077                                                                              684  SF13                                  3     ∞  0.500                                                          ______________________________________                                    

Various aberrations of this objective optical system are shown in FIG.10 and the wave aberrations are shown in FIG. 11.

In TABLE 3, it can be obtained almost the same efficiency when r₁ =r₃=500 is given.

EXAMPLE 2

FIG. 12 shows EXAMPLE 2 of the objective optical system. The numericalvalue construction of the chromatic aberration correcting element isshown in Table 4. The objective lens and the cover glass of the opticaldisk is the same to that of EXAMPLE 1.

Various aberrations of this objective optical system are shown in FIG.13 and the wave aberrations are shown in FIG. 14.

                  TABLE 4                                                         ______________________________________                                        NA 0.55  f = 3.30  ω = 1.7°                                      surface                                glass material                         NO.   r      d       n.sub.780                                                                           n.sub.d                                                                              ν.sub.780                                                                       name                                   ______________________________________                                        1     ∞                                                                              0.080   1.72421                                                                             1.74077                                                                               684 SF13                                   2     2.200  1.600   1.72437                                                                             1.73400                                                                              1071 LaK09                                  3     ∞                                                                              0.500                                                            ______________________________________                                    

EXAMPLE 3

FIG. 15 shows EXAMPLE 3 of the objective optical system. The numericalvalue construction of the chromatic aberration correcting element isshown in Table 5. The objective lens and the cover glass of the opticaldisk is the same to that of EXAMPLE 1.

                  TABLE 5                                                         ______________________________________                                        NA 0.55  f = 3.30  ω = 1.7°                                      surface                                 glass material                        NO.   r        d      n.sub.780                                                                           n.sub.d                                                                             ν.sub.780                                                                        name                                  ______________________________________                                        1     ∞  1.600  1.68442                                                                             1.69350                                                                             1136  LaK08                                 2     -2.200   0.800  1.68443                                                                             1.69895                                                                              755  SF15                                  3     ∞  0.500                                                          ______________________________________                                    

Various aberrations of this objective optical system are shown in FIG.16 and the wave aberrations are shown in FIG. 17.

EXAMPLE 4

FIG. 18 shows EXAMPLE 4 of the objective optical system. The numericalvalue construction of the chromatic aberration correcting element isshown in Table 6. The objective lens and the cover glass of the opticaldisk is the same to that of EXAMPLE 1.

Various aberrations of this objective optical system are shown in FIG.19 and the wave aberrations are shown in FIG. 20.

                  TABLE 6                                                         ______________________________________                                        NA = 0.55  f = 3.31  ω = 1.7°                                    surface                             glass material                            NO.   r         d      n.sub.780                                                                             ν.sub.780                                                                       name                                      ______________________________________                                        1     ∞   1.50   1.82195 875  LaSF05                                    2     -3.000    0.70   1.82484 553  SFL03                                     3     ∞   0.20                                                          ______________________________________                                    

EXAMPLE 5

FIG. 21 shows EXAMPLE 5 of the objective optical system. The numericalvalue construction of the chromatic aberration correcting element isshown in Table 7. The objective lens and the cover glass of the opticaldisk is the same to that of EXAMPLE 1.

Various aberrations of this objective optical system are shown in FIG.22 and the wave aberrations are shown in FIG. 23.

                  TABLE 7                                                         ______________________________________                                        NA = 0.55  f = 3.31  ω = 1.7°                                    surface                               glass material                          NO.      r      d        n.sub.780                                                                           ν.sub.780                                                                         name                                    ______________________________________                                        1        ∞                                                                              0.70     1.82484                                                                             553    SFL03                                   2        3.000  1.50     1.82195                                                                             875    LaSF05                                  3        ∞                                                                              0.20                                                          ______________________________________                                    

EXAMPLE 6

FIG. 24 shows EXAMPLE 6 of the objective optical system and concretenumerical value construction is shown in TABLE 8. The asphericalcoefficients of the objective lens are shown in TABLE 9. Variousaberrations of this objective optical system are shown in FIG. 22 andthe wave aberrations are shown in FIG. 23. Also, in order to determineaffection given to the chromatic aberration correcting element, variousaberrations and the wave aberrations by a single unit of the objectivelens are shown in FIGS. 27 and 28.

                  TABLE 8                                                         ______________________________________                                        NA = 0.55  f = 3.31  ω = 1.7°                                    surface                             glass material                            NO.   r         d      n.sub.780                                                                             ν.sub.780                                                                       name                                      ______________________________________                                        1     ∞   1.30   1.82195  875 LaSF05                                    2     -2.900    0.70   1.82484  553 SFL03                                     3     ∞   0.20                                                          4     2.116     2.00   1.53670 1507                                           5     -7.278                                                                  ______________________________________                                    

                  TABLE 9                                                         ______________________________________                                                 4th surface                                                                             5th surface                                                ______________________________________                                        K          -0.5086E + 00                                                                             -0.9722E + 00                                          A.sub.4     0.5580E - 04                                                                              0.1344E - 01                                          A.sub.6    -0.1938E - 04                                                                             -0.2130E - 02                                          A.sub.8     0.3046E - 04                                                                              0.1502E - 03                                          .sup. A.sub.10                                                                           -0.1039E - 04                                                                              0.2659E - 05                                          .sup. A.sub.12                                                                            0.0000E + 00                                                                              0.0000E + 00                                          ______________________________________                                    

EXAMPLE 7

FIG. 29 shows EXAMPLE 7 of the objective optical system, concretenumerical value construction is shown in TABLE 10 and the coefficient ofthe aspherical surface of the objective lens is shown in TABLE 11.Various aberrations of this objective optical system are shown in FIG.30 and the wave aberrations are shown in FIG. 31. Also, in order todetermine affection given to the chromatic aberration correctingelement, various aberrations and the wave aberrations by a single unitof the objective lens are shown in FIGS. 32 and 33.

                  TABLE 10                                                        ______________________________________                                        NA = 0.55  f = 3.30  ω = 1.7°                                    surface                             glass material                            NO.   r         d      n.sub.780                                                                             ν.sub.780                                                                       name                                      ______________________________________                                        1     ∞   1.30   1.78705  880 LaSF02                                    2     -3.600    0.70   1.78565  601 SFL6                                      3     ∞   0.20                                                          4      1.883    2.24   1.43107 1461                                           5     -3.732                                                                  ______________________________________                                    

                  TABLE 11                                                        ______________________________________                                                 4th surface                                                                             5th surface                                                ______________________________________                                        K.sup.     -0.5627E + 00                                                                             -0.4708E + 01                                          A.sub.4    -0.1402E - 03                                                                             0.2011E - 01                                           A.sub.6    -0.6290E - 04                                                                             -0.5946E - 02                                          A.sub.8     0.4537E - 04                                                                             0.9448E - 03                                           .sup. A.sub.10                                                                           -0.2548E - 04                                                                             -0.6470E - 04                                          .sup. A.sub.12                                                                            0.0000E + 00                                                                              0.0000E + 00                                          ______________________________________                                    

EXAMPLE 8

FIG. 34 shows EXAMPLE 8 of the objective optical system, concretenumerical value construction is shown in TABLE 12 and the coefficient ofthe aspherical surface of the objective lens is shown in TABLE 13. Inthis example, first and third surfaces are not plane.

                  TABLE 12                                                        ______________________________________                                        NA = 0.55  f = 3.30  ω = 1.7°                                    surface                             glass material                            NO.   r         d      n.sub.780                                                                             ν.sub.780                                                                       name                                      ______________________________________                                        1     50.000    1.50   1.82195  875 LaSF05                                    2     -2.822    0.70   1.82484  553 SFL03                                     3     50.000    0.10                                                          4     2.089     2.00   1.53670 1507                                           5     -6.770                                                                  ______________________________________                                    

                  TABLE 13                                                        ______________________________________                                                 4th surface                                                                             5th surface                                                ______________________________________                                        K.sup.     -0.4168E + 00                                                                             -0.5220E + 00                                          A.sub.4    -0.9556E - 03                                                                              0.1663E - 01                                          A.sub.6    -0.1979E - 03                                                                             -0.3824E - 02                                          A.sub.8     0.3396E - 05                                                                              0.5343E - 03                                          .sup. A.sub.10                                                                           -0.1894E - 04                                                                             -0.3071E - 04                                          .sup. A.sub.12                                                                            0.0000E + 00                                                                              0.0000E + 00                                          ______________________________________                                    

Various aberrations of this objective optical system are shown in FIG.35 and the wave aberrations are shown in FIG. 36.

EXAMPLE 9

FIG. 37 shows EXAMPLE 9 of the objective optical system, and concretenumerical value construction is shown in TABLE 14. In this example, aglass lens of 3 piece structure is used as the objective lens, and theoptical system has two chromatic aberration correcting elements.

                  TABLE 14                                                        ______________________________________                                        NA = 0.55  f = 3.72  ω = 1.5°                                    surface                                                                       NO.      r         d        n.sub.780                                                                           nd      ν.sub.780                        ______________________________________                                        1        ∞   0.80     1.68443                                                                             1.69895  755                                2        2.850     1.50     1.68442                                                                             1.69350 1136                                3        ∞   1.00                                                       4        ∞   0.80     1.68443                                                                             1.69895  755                                5        2.850     1.50     1.68442                                                                             1.69350 1136                                6        ∞   1.00                                                       7        9.066     1.00     1.79250                                                                             1.80400                                     8        -29.920   0.55                                                       9        -4.080    1.74     1.78565                                                                             1.80518                                     10       -4.080    0.08                                                       11       3.120     1.07     1.86890                                                                             1.88300                                     12       7.118     1.96                                                       ______________________________________                                    

Various aberrations of this objective optical system are shown in FIG.38 and the wave aberrations are shown in FIG. 39. Also, in order todetermine affection given to the chromatic aberration correctingelement, various aberrations and the wave aberrations by a single unitof the objective lens are shown in FIGS. 40 and 41. FIGS. 42 and 43 showexamples wherein a hologram lens is used as the objective lens. As thehologram lens is a lens utilizing diffraction, a moving amount(chromatic aberration on the axis) CA of the light converged positionwith respect to the wavelength fluctuation can be expressed as follows;

    CA=-f·(Δλ/λ)

wherein the focal length is represented by f, the central wavelength touse by λ, and the wavelength fluctuation by Δλ. That is, the movement ofthe converged position with respect to the wavelength fluctuation of 1nm at the wavelength of 780 nm becomes -f·(1/780) nm.

On the contrary, in the case of an ordinary lens utilizing refraction,as it becomes CA=-f·{Δn/(-1+n)}, its value becomesf·(1/10000)˜f·(1/25000).

Therefore, the generating amount of the chromatic aberration of thehologram lens is about 30 times of the lens utilizing refraction and thecharacter (plus or minus) becomes inverted. Owing to the foregoing, inorder to use the chromatic aberration correcting element in theabove-mentioned respective embodiments in order to use the hologramlens, it is necessary to arrange about 30 pieces of chromatic aberrationcorrecting element.

FIG. 42 shows a case where a plane hologram lens is used, while FIG. 43shows a case where a curved hologram lens is used.

By the way, as the chromatic aberration correcting element of theabove-mentioned EX.1 to EX. 9 is formed of a positive lens and anegative lens cemented together and both surfaces thereof are formed ina surface having no power. In this case, as the chromatic aberrationcorrecting effect is exhibited only at the cemented surfaces, it isnecessary that a difference of Δn/Δλ between the positive and negativelenses is made large and the radius of curvature of the cementedsurfaces is made small.

However, when the difference of Δn/Δλ between the positive lens and thenegative lens is large, the aberration at the peripheral portion becomeslarge, and when the radius of curvature of the cemented surfaces becomessmall, the effective aperture becomes small in order to obtain the edgethickness and the diameter of the effective luminous flux becomes large.As a result, the effective aperture of the luminous flux becomesdifficult to be obtained large.

When the generating amount of the aberration at the cemented surfaces isreviewed based on the S1 which is a coefficient of the ternary sphericalaberration, it can be expressed as follows; ##EQU2## wherein therefractive index of the lens on the light incident side is representedby n_(p), the refractive index of the lens on the outgoing side byn_(n), the radius of curvature of the cemented surfaces by r_(m), theincident height of the paraxial ray of light by h, the inclination ofthe paraxial light of the lens on the light incident side by β_(p), andthe inclination of the paraxial light of the lens on the outgoing sideby β_(n).

However, from the following relations; ##EQU3## if (Δn)² →0 is given, itis guided to the following relation; ##EQU4## and provided βp→0, h→1, itis further guided to the following relation; ##EQU5##

From the above-mentioned relation, it can be understood that thespherical aberration amount is proportional to the third power of thecurvature and is proportional to Δn.

On the other hand, as the chromatic aberration correcting effect dependson the number of surfaces having the curvature and the curvature, thechromatic aberration correcting surfaces are separated into twoportions, and the radius of curvature of each surface is made twice inorder to compare with the case where the chromatic aberration correctingsurface is not separated. As a result, presuming that the chromaticaberration correcting amount is the same, the generating amount of thespherical aberration can be restrained to 1/4.

Therefore, in the following EXAMPLES 10-12, the chromatic aberrationcorrecting element is formed of three lenses cemented together and thechromatic aberration correcting surfaces are separated and disposed intwo places.

Each end face of the chromatic aberration correcting element is formedin a surface having almost no power and constructed in such to generateonly the chromatic aberration without having power.

According to such construction, when compared with a chromaticaberration correcting element which has only one cementing surface, thesame amount of chromatic aberration correction can be achieved by 1/4spherical aberration generating amount. Therefore, when compared withthe case where the cemented surface is only one, the allowable width ofthe value of Δn is widened and therefore the width of selection of acombination of glass materials can be widened.

Furthermore, as the radius of curvature of the cemented surface islarge, the edge thickness of the positive lens can be obtained even whenthe effective aperture is made large. If the effective aperture islarge, even when the optical axis of the objective lens is brought outof the optical axis of the chromatic aberration correcting element, thepossibility for occurring the eclipse of the luminous flux is small.

Although the above explanation was made based on the ternary aberrationand the change of aberration which the ray of light near the opticalaxis has is found, the objective optical system having only one cementedsurface, when taking into consideration the affection of the high orderaberration, generates ten times or more of aberration with respect tothe ray of light passing the peripheral portion of the chromaticaberration correcting element when compared with one having the cementedsurface split into two portions.

By the way, at the time when the optical characteristic of the objectivelens system is evaluated, it is necessary to take into consideration theaffection of an adhesive used for cementing. An adhesive which is usedfor attaching optical parts such as ordinary lens, etc. has a refractiveindex of about 1.5˜1.6. When the refractive index of a glass material tobe cemented is different from the refractive index of the adhesive, alight refraction is occurred on that surface and an aberration isgenerated. As the aberration amount is proportional to the generatingamount of the aberration on the front and rear surfaces of the adhesivelayer, it is inversely proportional to the radius of curvature of thecemented surface and is proportional to the difference of the refractiveindex between the glass material and the adhesive.

The difference of the refractive index between the adhesive and theglass material becomes 0.1 or more because no adhesive of a highrefractive index is available at present and no combination of glassmaterials having a large difference of Δn/Δλ at a low refractive index.

For example, when an adhesive layer having a refractive index of 1.54000and a thickness of 0.01 mm on the cementing surface of the chromaticaberration correcting element of EXAMPLE 1, each aberration is varied asshown in FIG. 44. When the thickness of the adhesive layer is 0, theaberrations generated on the front and rear surface of the adhesivelayer are offset. However, when the adhesive layer is thick, theincident height of light between the front and rear surfaces is variedand the aberrations on the front and rear surfaces of the adhesive arenot completely offset. As a result, the aberration is generated and aproblem arises.

As the chromatic aberration correcting element of EXAMPLE 1 has only onecemented surface, it is indispensable to make the radius of curvature ofthe cemented surface small in order to correct the chromatic aberration.Therefore, when there is a difference between the refractive index ofthe adhesive and the refractive index of the glass material, the degreefor generating the aberration with respect to the change of thickness ofthe adhesive layer is large. By comparing with FIG. 10 in which theaffection by the adhesive is not taken into consideration, there can berecognized deterioration of efficiency on the peripheral portion.

When the cemented surface is split into two portions here, the chromaticaberration can be sufficiently corrected without making the radius ofcurvature small, and the aberration fluctuation on each cemented surfacewith respect to the change of thickness of the adhesive layer is small.Also, by making the two cemented surfaces into generally symmetricalshapes, the deterioration of efficiency can be restrained even when thethickness of the adhesive layer is irregular.

When the chromatic aberration correcting element is formed of threelenses or more cemented together, it is desirous to satisfy theconditions of the following relations (8) (9). ##EQU6## In the aboverelations, the symbolic characters have the following meanings.

Δn₁ /Δλ: gradient of change with respect to the wavelength of the ithlens

r₂ : radius of curvature of the cemented surfaces of the first andsecond lenses

r₃ : radius of curvature of the cemented surface of the second lens andthe third lens.

The relation (8) shows a relation similar to the relation (3) applied tothe lens formed of three lenses.

When escaped from the range of this relation, even if the curvature ofthe cemented surface is strengthened to correct the chromaticaberration, the change in degree of convergence or divergence on thecemented surface becomes excessively large when change in wavelength isgenerated. As a consequence, since the spherical aberration of highorder of the element itself becomes large, it becomes impossible that asufficient correction of the chromatic aberration is compatible with theprevention of other aberration deterioration.

The relation (9) shows the conditions for making the two cementingsurfaces into generally symmetrical shape.

As described previously, the aberration amount generating on thecemented surface is proportional to the generating amount of aberrationon the front and rear surfaces of the adhesive layer. Therefore, it isdesirous that the burden of the chromatic aberration is equalized andthe curvature of the both surfaces is made gentle. When the conditionsof the relation (9) are not satisfied, the effect for splitting thechromatic aberration correcting surface is small, and when theconditions are satisfied, the glass material can be made small in thegenerating amount of aberration on the interface with respect to theadhesive layer by satisfying the conditions.

Therefore, even if there is a small amount of irregularity in thicknessof the adhesive layer at the time when the surface is cemented,deterioration of efficiency can be restrained. When the condition ofr2=-r3 is satisfied, generation of the aberration becomes minimum.

EXAMPLE 10

FIG. 45 shows EXAMPLE 10 of an objective optical system according to thepresent invention, and concrete numerical value construction is shown inTABLE 15, and the aberration by this construction is shown in FIG. 46.In this EXAMPLE 10, as the thickness of the adhesive layer is also takeninto consideration, a surface number is put for each lens regarding thecemented surface. As the numerical value constructions for the objectivelens and the cover glass of the optical disk is the same to that ofEXAMPLE 1, description will be omitted in the following Table.

                  TABLE 15                                                        ______________________________________                                        NA 0.55   f = 3.30  ω = 1.7°                                     surface                                 glass                                 NO.   r       d       n.sub.780                                                                           n.sub.d                                                                             ν.sub.780                                                                        material                              ______________________________________                                        1     ∞ 1.400   1.72437                                                                             1.73400                                                                             1071  LaK09                                 2     -4.400  0.010   1.54000           adhesive                              3     -4.400  0.800   1.72421                                                                             1.74077                                                                              684  SF13                                  4      4.400  0.010   1.54000           adhesive                              5      4.400  1.400   1.72437                                                                             1.73400                                                                             1071  LaK09                                 6     ∞ 0.500                                                           ______________________________________                                    

FIG. 47 shows the aberration in a case where the adhesive layer is notprovided. From FIGS. 46 and 47, it will be understood that the variousaberrations are hardly changed depending on whether the adhesive layeris provided.

EXAMPLE 11

FIG. 48 shows EXAMPLE 11 of an objective optical system and the concreteconstruction of numerical values thereof is as shown in TABLE 16. FIG.49 shows the aberration when the affection of the adhesive according tothis construction is taken into consideration.

                  TABLE 16                                                        ______________________________________                                        NA 0.55  f = 3.30  ω = 1.7°                                      surface                                 glass                                 NO.   r       d       n.sub.780                                                                           n.sub.d                                                                             ν.sub.780                                                                        material                              ______________________________________                                        1     ∞ 0.800   1.72421                                                                             1.74077                                                                              684  SF13                                  2      4.400  2.000   1.72437                                                                             1.73400                                                                             1071  LaK09                                 3     -4.400  0.800   1.72421                                                                             1.74077                                                                              684  SF13                                  4     ∞ 0.500                                                           ______________________________________                                    

EXAMPLE 12

FIG. 50 shows EXAMPLE 12 of an objective optical system and the concreteconstruction of numerical values thereof is as shown in TABLE 17. FIG.51 shows the aberration when the affection of the adhesive according tothis construction is taken into consideration.

                  TABLE 17                                                        ______________________________________                                        NA 0.55  f = 3.30  ω = 1.7°                                      surface                                 glass                                 NO.   r       d       n.sub.780                                                                           n.sub.d                                                                             ν.sub.780                                                                        material                              ______________________________________                                        1     ∞ 1.400   1.73145                                                                             1.74100                                                                             1076  LaK011                                2     -4.400  0.800   1.73166                                                                             1.75000                                                                              621  SFS53                                 3      4.400  1.400   1.73156                                                                             1.74100                                                                             1076  LaK011                                4     ∞ 0.500                                                           ______________________________________                                    

In the above-mentioned respective embodiments, assuming that the centralwavelength in use is 780 nm, the construction has a satisfactoryefficiency at this wavelength. However, the application of the presentinvention is not limited to the above-mentioned wavelength but it canalso be applied to other wavelength ranges. Examples of a combination ofglass materials satisfying the above-mentioned conditions of the presentinvention in wavelengths having a central wavelength in use of otherthan about 780 nm are as follows;

In the following relation, n_(p) represents the refractive index of apositive lens, n_(n) the refractive index of a negative lens, and Δn/Δλthe gradient of a change with respect to the wavelength of therefractive index of each glass material.

    ______________________________________                                        <wavelength of 830 nm>                                                        positive lens LaSK02 (Ohara)                                                  n.sub.830 = 1.77419                                                                           Δn/Δλ = -3.3 × 10.sup.-5                             nm.sup.-1                                                     n.sub.d = 1.78650                                                                             ν.sub.d = 50.0                                             negative lens SFS54 (Minolta)                                                 n.sub.830 = 1.77372                                                                           Δn/Δλ = -6.0 × 10.sup.-5                             nm.sup.-1                                                     n.sub.d = 1.79850                                                                             ν.sub.d = 22.6                                             n.sub.p - n.sub.n = 47 × 10.sup.-5                                      (Δn.sub.p /Δλ - Δn.sub.n /Δλ)           × λ.sup.2 = 18.8 nm                                              n.sub.830: refractive index in the wavelength of 830 nm                       <wavelength of 670 nm>                                                        positive lens LaF04 (Ohara)                                                   n.sub.670 = 1.75145                                                                           Δn/Δλ = -5.6 × 10.sup.-5                             nm.sup.-1                                                     n.sub.d = 1.75700                                                                             λ.sub.d = 47.8                                         negative lens SFL14 (Ohara)                                                   n.sub.670 = 1.75224                                                                           Δn/Δλ = -9.4 × 10.sup.-5                             nm.sup.-1                                                     n.sub.d = 1.76182                                                                             λ.sub.d = 26.5                                         n.sub.p - n.sub.n = 79 × 10.sup.-5                                      (Δn.sub.p /Δλ - Δn.sub.n /Δλ)           × λ.sup.2 = 17.0 nm                                              n.sub.670: refractive index in the wavelength of 670 nm                       <wavelength of 632 nm>                                                        positive lens LaSK01 (Ohara)                                                  n.sub.532 = 1.75979                                                                           Δn/Δλ = -10.0 × 10.sup.-5                            nm.sup.-1                                                     n.sub.d = 1.75500                                                                             ν.sub.d = 52.3                                             negative lens SFS53 (Minolta)                                                 n.sub.532 = 1.75986                                                                           Δn/Δλ = -21.1 × 10.sup.-5                            nm.sup.-1                                                     n.sub.d = 1.75000                                                                             ν.sub.d = 25.1                                             n.sub.p - n.sub.n = -7 × 10.sup.-5                                      (Δn.sub.p /Δλ - Δn.sub.n /Δλ)           × λ.sup.2 = 31.3 nm                                              n.sub.532 : refractive index in the wavelength of 532 nm                      ______________________________________                                    

Next, the relation between each embodiment and each conditional relationwill be shown in the following TABLE 18.

                  TABLE 18                                                        ______________________________________                                        (1)      (2)    (3)     (4)   (5)  (6)   (7)  (8)                             ______________________________________                                        EX. 1 16.6   0.262  12.5  0.00023                                                                             ∞                                                                            ∞                                                                             ∞                                                                            --                            EX. 2 16.6   0.262  12.5  0.00023                                                                             ∞                                                                            ∞                                                                             ∞                                                                            --                            EX. 3  0.6   0.230  10.1  0.00084                                                                             ∞                                                                            ∞                                                                             ∞                                                                            --                            EX. 4 289    0.302  16.5  0.00352                                                                             ∞                                                                            ∞                                                                             ∞                                                                            --                            EX. 5 289    0.302  16.5  0.00352                                                                             ∞                                                                            ∞                                                                             ∞                                                                            --                            EX. 6 289    0.302  16.5  0.00352                                                                             ∞                                                                            ∞                                                                             ∞                                                                            --                            EX. 7 140    0.250  12.8  0.00178                                                                             ∞                                                                            ∞                                                                             ∞                                                                            --                            EX. 8 289    0.302  16.5  0.00246                                                                             17.7 15.2  15.2 --                            EX. 9  0.6   0.230  10.1  0.00084                                                                             ∞                                                                            ∞                                                                             ∞                                                                            --                            EX. 10                                                                              16.6   0.262  12.5  0.00044                                                                             ∞                                                                            ∞                                                                             ∞                                                                            -1.0                          EX. 11                                                                              16.6   0.262  12.5  0.00024                                                                             ∞                                                                            ∞                                                                             ∞                                                                            -1.0                          EX. 12                                                                              21.0   0.309  16.0  0.00057                                                                             ∞                                                                            ∞                                                                             ∞                                                                            -1.0                          ______________________________________                                    

By the way, above-mentioned optical systems are designed for reducingeffects of chromatic aberration. However, it is possible to design anobjective optical system which is used the changing of the convergingpoint due to the chromatic aberration positively. Standing this viewpoint, the change of the converging point is able to use for focusingservo instead of actuating an objective lens.

As change the wevelength of light from light source, converging point ischange. Therefor, when the out of focus is detected, light sorce drivercontrols wevelength of light as such that amount of out of focus isoffsetted by amount of change of converging point.

Particularly, when the objective optical system is designed as such thata relation between the changing amount of converging point and the shiftof wavelength is liner, control of wevelength is easy.

What is claimed is:
 1. An optical system of an optical informationrecording/reproducing apparatus including:a light source for emittingdivergent luminous flux; a collimator lens for collimating the luminousflux emitted from said light source; an objective lens for convergingthe luminous flux emitted from said light source onto a medium; a beamsplitter for splitting the luminous flux reflected by said medium from alight path directed to the light source and guiding the luminous flux toa light receiving system; a chromatic aberration correcting elementhaving substantially no power disposed between said objective lens andsaid beam splitter and adapted to correct a chromatic aberration of saidobjective lens; and means for independently actuating said objectivelens at least in an optical axis direction defined by said objectivelens; said chromatic aberration correcting element comprising a positivelens and a negative lens cemented together, each end face of saidchromatic aberration correcting element being formed with a generallyplanar surface.
 2. An optical system of an optical informationrecording/reproducing apparatus of claim 1, wherein said actuating meansindependently actuates said objective lens in said optical axisdirection and in the vertical direction with respect to said opticalaxis.
 3. An optical system of an optical informationrecording/reproducing apparatus of claim 1, which further includes ahead moved with respect to said medium, and in which said objective lensand said chromatic aberration correcting element are disposed on saidhead.
 4. An optical system of an optical informationrecording/reproducing apparatus of claim 3, wherein said actuating meansindependently actuates said objective lens in said optical axisdirection and in the vertical direction with respect to said opticalaxis.
 5. An optical system of an optical informationrecording/reproducing apparatus of claim 1, which further includes ahead moved with respect to said medium and in which said objective lensis disposed on said head and said chromatic aberration correctingelement is disposed outside said head.
 6. An optical system of anoptical information recording/reproducing apparatus of claim 5, whereinsaid actuating means independently actuates said objective lens in saidoptical axis and in the vertical direction with respect to said opticalaxis.
 7. An optical system of an optical informationrecording/reproducing apparatus of claim 1, which further includes ahead moved with respect to said medium and in which said light sourceportion, said objective lens, said chromatic aberration correctingelement, said light receiving system and said beam splitter are disposedwithin said head.
 8. An optical system of an optical informationrecording/reproducing apparatus of claim 7 wherein said actuating meansindependently actuates said objective lens in said optical axis and inthe vertical direction with respect to said optical axis.
 9. An opticalsystem of an optical information recording/reproducing apparatus ofclaim 1, wherein said objective lens and said chromatic aberrationcorrecting element are independently corrected in aberrations other thanthe chromatic aberration.
 10. The optical system of an opticalinformation recording/reproducing apparatus according to claim 1, saidchromatic aberration correcting element being movable independently ofsaid collimator lens.
 11. An optical system of an optical informationrecording/reproducing apparatus according to claim 1, said chromaticaberration correcting element satisfying the following relation:

    |n.sub.p -n.sub.n |×10.sup.5 <300

where n_(p) is the refractive index of a positive lens in a center of awavelength used; and n_(n) is the refractive index of a negative lens ina center of a wavelength used.
 12. An objective optical system of anoptical information recording/reproducing apparatus including:anobjective lens for converging a generally parallel luminous flux; achromatic aberration correcting element having substantially no powerfor correcting a chromatic aberration of said objective lens; and meansfor independently actuating said objective lens at least in an opticalaxis direction defined by said objective lens; said chromatic aberrationcorrecting element being formed of a positive lens and a negative lenscemented together, each end face of said chromatic aberration correctingelement being formed as a generally planar surface.
 13. An objectiveoptical system of an optical information recording/reproducing apparatusof claim 12, wherein said actuating means independently actuates saidobjective lens in the optical axis direction and in the verticaldirection with respect to said optical axis.
 14. An objective opticalsystem of an optical information recording/reproducing apparatus ofclaim 12, wherein said objective lens and said chromatic aberrationcorrecting element are independently corrected in aberrations other thanthe chromatic aberration.
 15. The objective optical system of an opticalinformation recording/reproducing apparatus according to claim 12,further comprising a head movable with respect to a medium, saidobjective lens and said chromatic aberration correcting element beingdisposed on said head.
 16. The optical system of an optical informationrecording/reproducing apparatus according to claim 12, said chromaticaberration correcting element satisfying the following relation:

    |n.sub.p -n.sub.n |×10.sup.5 <300

where n_(p) is the refractive index of a positive lens in a center of awavelength used; and n_(n) is the refractive index of a negative lens ina center of a wavelength used.